1. Field of the Invention
The present invention relates generally to the fields of image transfer and the photolithographic transfer of images to a semiconductor wafer, printed circuit board or other substrate, and more particularly, to an apparatus and method for selectively exposing a photosensitive layer of material to a patterned source of light without the need for a mask.
2. Description of the Relevant Art
Image transfer of complex patterns onto substrates such as semiconductor wafers, printed circuit boards, flat panel displays, and the like commonly employ the use of a photolithographic apparatus containing a light source, a system of lenses and/or mirrors and a photomask, mask or reticle. In common step-and-repeat, or step-and-scan systems, light moves from a light source, through a lens/mirror assembly, and through a patterned mask onto the substrate which is covered with a photosensitive polymer resist. The mask is a two dimensional stencil of the pattern to be transferred. Often, thirty or more different patterns, or layers, are transferred to a given substrate during the manufacturing process, requiring a different mask for each such layer. The mask manufacturing process has proven to be a costly and time-consuming process. The mask itself is easily damaged during everyday handling; accordingly, it is common to produce duplicate masks in order to replace masks that become damaged. Elimination of the various masks would save time and money in the manufacturing process.
The advantages inherent in eliminating the need for such exposure masks has long been recognized, and those skilled in the art have explored mask free lithography in depth. Current directwrite methods are known, such as laser, electron or ion beam lithography, wherein a fine beam of light is selectively steered to shine on each point of the photosensitive film that needs to be exposed. For example, in U.S. Pat. No. 5,451,489 to Leedy, an electron beam is used to selectively expose a photoresist layer on a semiconductor wafer without the use of a mask. Similarly, in U.S. Pat. No. 5,109,149 to Leung, a laser beam is used in conjunction with a polygonal mirror, a beam expander, and a lens to selectively direct light onto the surface of a wafer mounted on an X-Y axis motorized table. However, these direct-write techniques have been proven to be too slow for economic commercial use.
In U.S. Pat. No. 5,691,541 to Ceglio, et al., a lithography system is described wherein a programmable array of "light switches", in the form of an array of digital micro-mirror devices, is provided to control the passage of light from a source to a photosensitive layer to be exposed. Each micro-mirror is either deflected through an angle to form a dark portion of the pattern, or undeflected to form a bright portion of the pattern. However, the device described by Ceglio, et al. is dependent upon the proper alignment of many small reflecting mirrors in order to reflect the desired image to the substrate. Reflective aberrations and mirror mis-alignment cause inaccuracies in the image that is projected onto the substrate.
In U.S. Pat. No. 5,781,331 to Carr, et al., an optical micro-shutter array is described that can be produced using known semiconductor fabrication processes. The disclosed optical shutter includes an aperture plate positioned in a light path, the aperture plate having an array of apertures formed therein. A series of microcantilevers are used to selectively cover the array of apertures, each microcantilever being associated with one of the apertures. Carr, et al. describe such microcantilevers as preferably being formed of two layers of material having different thermal coefficients of expansion, and preferably being thermally-actuated, although Carr, et al. also state that piezoelectric and electrostatic-originating forces may also be employed. When a microcantilever is heated by passing an electrical current through an associated resistor, the microcantilever curls away from the associated aperture, thereby allowing light to pass through such aperture. One disadvantage of using such microcantilevers is that their up and down curling/flexing motion tends to cause unwanted interference between two adjacent microcantilevers; if a first microcantilever is trying to curl up away from its aperture, and a second adjacent microcantilever is trying to move down toward its adjacent aperture, then the two microcantilevers may contact each other.
U.S. Pat. No. 5,808,384 to Tabat describes a micromechanical actuator that can be formed on substrates using lithographic processing techniques. Tabat describes such devices as being useful for, among other things, forming optical switches. A plunger having two magnetic heads is supported within a gap of a magnetic core to which an electrical coil is coupled. A pair of springs bias the plunger to a central position. The application of electrical current to the electrical coil moves the plunger back and forth in a linear movement depending upon the direction of current flow. However, the necessity of having the actuators pass through the core of the electrical coil places restrictions on how close two or more of such actuators can be positioned relative to each other. Moreover, the need to form spring-like biasing members within the substrate further complicates the fabrication of such devices.
Accordingly, it is an object of the present invention to provide a maskless photoresist exposure system which eliminates the need for masks in order to selectively expose photosensitive layers applied to semiconductor wafers, printed circuit boards, or other substrates.
Another object of the present invention is to provide such a maskless photoresist exposure system capable of using a conventional photolithographic light source and avoiding the need for lasers, electron beams, or ion beams.
Still another object of the present invention is to provide such a maskless photoresist exposure system which operates quickly enough to prove economically feasible for commercial use.
A further object of the present invention is to provide such a maskless photoresist exposure system which avoids the need for precise alignment of small mirrors in order to produce a patterned light image.
A still further object of the present invention is to provide such a maskless photoresist exposure system which includes a series of optical shutters that can be disposed closely proximate one another to form adjacent pixels of light, yet wherein movement of one such shutter does not interfere with movement of shutters adjacent thereto.
A yet further object of the present invention is to provide such a maskless photoresist exposure system wherein the aforementioned series of optical shutters can themselves be formed using known photolithographic semiconductor processing techniques.
Yet another object of the present invention is to provide such a maskless photoresist exposure system wherein the aforementioned series of optical shutters need not themselves pass through the core of an electrical coil.
Still another object of the present invention is to provide a method of performing mask free photolithography.
An additional object of the present invention is to provide a novel method of forming an electrical coil, suitable for use in forming an electromechanical actuator, on a semiconductor substrate using known semiconductor wafer processing techniques.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.